1. Field of the Invention
The present invention relates to an energy device and a method for producing the same.
2. Description of the Related Art
A lithium ion secondary battery includes a negative collector, a negative active material, an electrolyte, a separator, a positive active material, and a positive collector as main components. The lithium ion secondary battery plays an important role as an energy source for mobile communication equipment and various kinds of AV equipment. Along with the miniaturization and the enhanced performance of equipment, the miniaturization and the increase in energy density of the lithium ion secondary battery are proceeding. Thus, a great amount of efforts are being put into improving each element constituting the battery.
For example, JP8(1996)-78002A discloses that an energy density can be increased by using, as a positive active material, an amorphous oxide obtained by melting mixed powder of a particular transition metal oxide with heating, followed by rapid cooling.
Furthermore, JP2000-12092A discloses that a battery capacity and a cycle life can be enhanced by using a transition metal oxide containing lithium as a positive active material, using a compound containing silicon atoms as a negative active material, and setting the weight of the positive active material to be larger than that of the negative active material.
Furthermore, JP2002-83594A discloses that an amorphous silicon thin film is used as a negative active material. Due to the use of the amorphous silicon thin film, a larger amount of lithium can be absorbed, compared with the case of using carbon, so that an increase in capacity is expected.
However, in the case where the amorphous silicon thin film is used as a negative material for a lithium ion secondary battery, although the amorphous silicon thin film absorbs a large amount of lithium, the expansion/contraction amount of the amorphous silicon thin film during charging/discharging also is large. Therefore, a problem of maintaining cycle characteristics arises. The amorphous silicon thin film can be formed by a vacuum film-forming process such as sputtering, vapor deposition, or the like. In order to increase the capacity, the amorphous silicon thin film that is a negative active material needs to be thick to some degree in accordance with the thickness of a positive electrode. However, when the thickness of the amorphous silicon thin film is set to be large, although the battery capacity is increased, cycle characteristics tend to be degraded. Thus, it has been difficult to satisfy both a large capacity and cycle characteristics.